One indication of a patient's condition during respiration support is the status of the patient's work of breathing (WOB). Work of breathing may be defined as the work associated with inflating the patient's lungs during a breathing cycle. During respiration support, a ventilator provides at least a portion of the total work of breathing for the patient. The total work of breathing (WOBTOTAL) may generally be defined as the sum of the work of breathing provided by the patient (WOBPATIENT) and the work of breathing support provided by the ventilator (WOBVENTILATOR).
The work of breathing provided by the patient (WOBPATIENT) may be approximated as the sum of two components: an elastic WOB component (WOBPATIENT-ELASTIC) and a resistive WOB component (WOBPATIENT-RESISTIVE). The elastic WOB component is generally defined as the work required to overcome the elastance of the patient's respiratory system, while the resistive component is generally defined as the work required to overcome the airway resistance of the patient's respiratory system.
Elastance may generally be defined in terms of the elastic properties of the lung and chest, or the forces associated with expanding the lung. In particular, the degree of stiffness of the lung-chest region may be referred to as the elastance of the respiratory system. The elastance of the respiratory system may also be discussed in terms of compliance, which may be defined as the inverse of elastance. Generally, the easier it is to stretch the lung-chest region.
Resistance forces, or the non-elastic forces at work in the breathing cycle, are the forces associated with moving air through a patient's airways. Lung resistance may be at least partially defined by a patient's physiological conditions. For example, patients suffering from asthma typically experience muscular constriction of the bronchi. Such patients may also experience swelling of the bronchial mucosa. The work required to achieve a particular amount of air flow through the breathing passageways generally increases in proportion to the severity of constriction. In some ventilation systems, flow and pressure sensors are used to compute estimates of the patient's resistance and compliance.
One or more WOB values, e.g., the total WOB, the patient's WOB (WOBPATIENT), the ventilator's WOB (WOBVENTILATOR), the elastic WOB component (WOBPATIENT-ELASTIC), and/or the resistive WOB component (WOBPATIENT-RESISTIVE) may be determined and/or monitored in various manners. For example, a patient's WOB may be determined from either measured or estimated values relating to the patient's respiratory physiology by applying direct or indirect approaches and following established algorithms. Measured values may be obtained more directly by invasive procedures, e.g., procedures that require the installation of an esophageal balloon. Such techniques are typically invasive and require specialized skill. Thus, outside of the research setting it may be undesirable to obtain WOB measures employing such invasive means.
In a pressure assisted ventilation (PAV) system, the patient's work of breathing (WOBPATIENT), the elastic WOB component (WOBPATIENT-ELASTIC), and/or the resistive WOB component (WOBPATIENT-RESISTIVE) may be estimated by inputting measurements from various sensors into the breathing algorithms. In PAV ventilation, the patient is supplied with continuous pressure assistance throughout an inspiratory effort and in direct proportion to the moment-to-moment inspiratory effort. Typically, none of the instantaneous inspiratory pressure, the instantaneous flow, or the resulting volume are set by the caregiver. Because the PAV breathing algorithm harmoniously links the ventilator to the patient, the patient effectively “drives” the ventilator. By appropriately setting the value of the proportionality (% support) control, the caregiver may effectively partition the total WOB between the patient (WOBPATIENT) and the ventilator (WOBVENTILATOR).
The values of the patient's lung-chest compliance and lung resistance may be continuously estimated and inserted into the PAV breathing algorithm in order for the algorithm to function properly. These estimates may be made automatically by the ventilator and fed back to the breathing algorithm as perhaps better fits the needs of the patient in intensive care, whereas manual techniques may be used to estimate the values for more stable patients, e.g., in a home setting.